It has already been suggested to improve the performance of certain internal combustion engines, to shape their exhaust pipe with a diverging section then a converging section as seen in the direction of the gas flow, such a device being sometimes referred to as an "extractor". However, because such devices are only fully operational in a small operating range of the engine, i.e. in a small range of rotational speeds, they do not enable to satisfy the imposed requirements of noise level limitations for the exhaust of the engine which is equipped with them.
However, there exists, in other respects, a large number of silencer devices for internal combustion engines of the type comprising deflectors or absorbing materials, such silencers introduce by their construction perturbations in the flow of exhaust gases, which generally results in a reduction in performance of the engines with which they are associated.